Recent changes in compressor design have suggested a need for changes in how motor temperature is controlled. Past methods for control of motor temperature have used a Proportional Integral Derivative (PID) control system to control the system motor temperature. The traditional PID control system monitors the temperature of the motor housing to control the system motor temperature. The traditional PID control system is used to control a valve which provides a coolant into the motor to cool the motor when the temperature exceeds a preselected set point. In one system, the motor is used to operate a compressor, and the coolant is refrigerant. When the valve is an electronic expansion valve (EEV), the valve operates to expand liquid refrigerant, lowering the pressure and the temperature of the refrigerant, so that a mist enters the motor for purposes of cooling. The PID control system monitors the temperature of the motor housing to determine whether a preselected set point is reached, and signals for an opening of the valve when the set point is reached, and closes the valve, thereby restricting the flow of cooling fluid into the motor when the temperature is below the set point.
Recent advances in compressor design have resulted in larger compressors. These larger compressors have larger motors with resulting larger motor housings. The larger motors also have resulted in increased heat generated by the motors, while the additional mass that has been added to the larger motor housings has increased the heat capacity of the motor systems. In addition, some of these compressor designs have incorporated electromagnetic (EM) bearings to balance the rotor during operation, which generate additional heat within the motor housing. In some designs, the materials used for the motor housings have changed. So, in those designs in which larger cast iron motor housings have been substituted for smaller aluminum or aluminum alloy motor housings, not only has the mass of the motor housings changed, but the thermal conductivity of the housings has changed, the aluminum and aluminum alloy and copper and copper alloy motor housings having a higher thermal conductivity than the cast iron motor housings. Generally, cast iron also has a lower specific heat capacity than aluminum, by a factor of two. This means that for a system having the same material mass and the same heat input, a cast iron housing will increase in temperature at about twice the rate as an aluminum housing. Clearly, systems having larger motors, larger motor housings made from materials with lower thermal conductivity and that incorporate additional sources of heat, such as EM bearings will be less responsive to cooling based on changes in motor housing temperature. As used herein, the combination of thermal conductivity, component (motor housing) mass, specific heat capacity of the component mass and heat generated within the component is used herein to refer to the thermal inertia of the system. Recent compressor advances utilizing larger, cast iron motor housings and larger motors are defined herein as high thermal inertia systems because of their slower rate of heating and cooling, and may also include EM bearings, while prior art systems utilizing aluminum, aluminum alloy, copper or copper motor housings, smaller motors utilizing small cast iron motor housings and mechanical bearings are defined herein as low thermal inertia systems, which tend to be more responsive to cooling, when identical cooling designs are utilized in the high inertia and low inertia system. When two systems have the same mass but utilize different materials for the motor housing, such as cast iron and aluminum alloy, the aluminum alloy system, being the low thermal inertia system, will respond more quickly to temperature changes when identical cooling systems are utilized.
As motor sizes increase while more cost effective materials in the form of high thermal inertia materials are incorporated into the design, what is needed is a control scheme that is more responsive to changes in motor temperature in a system having a high thermal inertia than current control schemes used in low thermal inertia systems.